An increase in flavours—such as aroma-producing substances—in tobacco can generate a desirable taste when the tobacco is smoked. Such flavours can be obtained, for example, from amino acids which are subjected to Maillard reactions. Methional (3-(methylthio)propanal) is a flavour compound responsible for a “baked potato” aroma. The production of methional can be induced thermally where methional originates from methionine and methionine derivatives. The Strecker degradation of methionine involves interaction with alpha-dicarbonyl compounds, which are intermediates in the Maillard reactions, and result in the formation of methional. Various methods for increasing the amount of free methionine in plants have been developed, for example, exogenous amino acids can be added to plants. However, the use of exogenously added amino acids results in a significant increase in production cost, as well as safety and regulatory concerns.
The biosynthesis of methionine and threonine are both linked to the aspartate pathway. In plants, their biosynthetic pathways diverge at the level of O-phosphohomoserine (OPH). The enzymes cystathionine gamma-synthase (CGS) and threonine synthase (TS) compete for the common substrate O-phosphohomoserine. Free methionine levels can potentially be increased by over-expressing or inhibiting expression of enzymes involved in the aspartate biosynthetic pathway.
One possible approach is to overexpress cystathionine gamma-synthase. Another approach is to decrease the expression of threonine synthase. However, to date all such efforts directed to alter threonine synthase genes resulted in phenotypes that adversely affected the entire plant. Bartlem et al. (2000) Plant Physiol., 2000, 123:101-110) describes mutations in the threonine synthase gene of Arabidopsis plants. The disclosed mutants carrying a single base pair mutation within the gene encoding threonine synthase exhibited an over-accumulation of methionine and a markedly reduced level of threonine. However, the disclosed mutants of Arabidopsis suffered from reduced growth compared with that of the wild type. The stunted growth can be rescued only upon threonine or isoleucine addition.
Zeh et al. (2001) Plant Physiol., 2001, 127:792-802 discloses transgenic potato plants prepared by an antisense transgenic approach using the constitutive cauliflower mosaic virus 35S promoter. Whilst the disclosed transgenic potato plants exhibited high levels of methionine, they also suffered from reduced growth as compared with that of the wild type plants.
Avraham et al. (2005) Transgenic Research, 2005, 14: 299-311 describes transgenic Arabidopsis plants that are prepared by an antisense transgenic approach. Whilst, the disclosed transgenic plants exhibited an increased level of methionine, they suffered from severely abnormal phenotypes, including considerable growth retardation, reduced rosette leaf size and chlorotic leaves.
There is a need for plants—such as tobacco plants—that combine an increased level of free methionine while maintaining certain agronomically desirable properties—such as growth rate and overall size of the plants—and without requiring the addition of any exogenous ingredients. It is an object of the present invention to satisfy this need.